1. Filed of the Invention
The present invention concerns optical resolution of DL-cysteine, or, in other words, a method of producing optically active cysteine or cystine.
2. State of the Art
L-cysteine is produced nowadays mainly by extracting it from natural raw materials. Because this way of production cannot meet rapidly increasing demand, other ways of industrial production such as chemical synthesis or fermentation methods have been sought. In cases according to the synthesis, the product will be in DL-form, and therefore, it should be optically resolved to obtain L-cysteine.
General methods of optically resolving DL-amino acids known to date are preferential crystallization, resolution with enzymes, and diastereomer resolution. As far as DL-cysteine is concerned, because it is chemically rather unstable and racemization reaction is difficult to occur, there has been established no method of commercially practicable optical resolution by the preferential crystalization or diastereomer resolution.
With respect to the method of resolution using the enzyme, Japanese Patent Disclosure No. 29293/1976 describes a method of effecting acylase to S-alkyl- or aryl-mercapto-N-acyl-DL-cysteine to obtain optically active S-alkyl- or aryl-mercapto-cysteine, and reducing the latter to form optically active cysteine. The method cannot be said advantageous for commercial practice, because the method not only uses the enzyme which is an organism substance, but also requires multiple steps of N-acylation, S-alkyl- or aryl-mercaptization, and further, dealkyl- or dearyl-mercaptization.
On the other hand, as an example of optically resolving certain DL-amino acids with optically active resolving agent, a method using mandelic acid was disclosed in Japanese Patent Disclosure No. 119844/1978. Also, use of L-phenyl alanine for optical resolution of mandelic acid was reported in J. Chem. Soc. Jpn. 92 (11) 999 (1971).